Ozone reactions in water can be classified as direct and indirect reactions. Direct reactions are selective reactions and their rate constants are very low, generally in the range of 1.0-103 M−1s−1. Indirect ozone reactions are via free hydroxyl radical oxidation, and the reactions of hydroxyl radicals with organic compounds are non-selective and have high rate constants within 108-1010 M−1s−1. In terms of the features of the two reaction manners of ozone and organic compounds in water, indirect ozone reactions are of great significance, especially in the treatment of wastewater which is difficult to be degraded. Indirect reactions of ozone are radical reactions which follow the laws of free radical reactions, including chain initiation, chain propagation and chain termination. Typical hydroxyl radicals scavengers in water treatment have been summarized in “Ozonation of Water and Wastewater: A Practical Guide to Understanding Ozone and its Applications”, Christiane Gottschalk et al (Eds.), mainly encompassing HCO3−, CO32−, PO34−, humic acid, aromatic compounds, isopropyl alcohol, TBA and so forth, wherein, HCO3− and CO32−, especially CO32−, are strong hydroxyl radical scavengers often encountered. The equations for the reactions of hydroxyl radicals with them are depicted as follows:OH.+CO32−→OH−+CO3.−k=4.2×108 M−1s−1 OH.+HCO3−→OH−+HCO3.k=1.5×107 M−1s−1 
Carbonate radicals are non-reactive with organic compounds (in the presence of hydrogen peroxide, carbonate radicals are capable of converting hydrogen peroxide into hydroxyl radicals).
Rate constants for reactions of hydroxyl radicals with partial low-molecular-weight organic acids (or acid anions) and inorganic acid anions are shown in Table 1. As can be seen from Table 1, the rate constant of the reaction between carbonate anions and hydroxyl radicals is far higher than that between these low-molecular-weight organic acids (or acid anions) and hydroxyl radicals, and the rate constant for the reaction between bicarbonate anion and hydroxyl radical is also higher than the rate constant of the reaction between oxalic acid (or oxalate anion) and hydroxyl radicals. In this case, a great amount of hydroxyl radicals yielded by catalytic decomposition of ozone is wasted by carbonate and bicarbonate anions, retarding reactions with low-molecular-weight organic acids. The process of treatment of wastewater by base-catalyzed ozonation is a mineralization process of organic compounds. With the proceeding of reactions, more and more carbonate and bicarbonate anions accumulate in water, which increasingly retards the reaction between ozone and organic compounds, resulting in the rise of treatment cost. The rate constant of the reaction between phosphate and hydroxyl radical approximates to that of between oxalic acid and hydroxyl radical, is lower than that of most of the low-molecular-weight organic acids, indicating a far lower capability to reduce the utilization efficiency of ozone than carbonate anion.
TABLE 1Rate constants of reactions of hydroxyl radicals with partial low-molecular-weight organic acids (or acid anions) and inorganic acid anionsReactantsk (×107) (M−1s−1)Formic acid13Acetic acid1.6Acetate anion8.5Oxalic acid0.14Oxalate anion0.77Malonic acid2Carbonate anion42Bicarbonate anion1.5Dihydrogen phosphate<0.01anionHydrogen phosphate anion0.22
Regarding how to remove carbonates in wastewater which act as hydroxyl radical scavengers yielded during ozonation, a method of pH sequential ozonation processes which are carried out at alternating time periods of acid and basic pHs is described in “Ozone Reaction Kinetics for Water and Wastewater Systems,” Fernando Juan Beltran Novillo, Ph.D. When the ozonation proceeds under a basic condition, mineralization of organic compounds produces carbonates. Upon the accumulation of carbonates to a certain extent in wastewater, pH is adjusted to an acid pH, which removes carbonates from water in the form of carbon dioxide. After removal of carbonates in water, pH is increased (by adding basic) again, and then indirect reactions are conducted. Alternating cycles in such way achieve the objective of removing COD in water. Such method for removing hydroxyl radical scavenger carbonates entails complicated operation procedures, consumes a large amount of acids and bases and requires high costs. Furthermore, the carbon dioxide has certain solubility in water after all, giving rise to a portion of carbon dioxide left in water. Particularly, in the case of low-temperature reactions, the residual amount of carbon dioxide is greater (e.g. the solubility of carbon dioxide is 2.318 g/1000 g water under a pressure sum of 0.101 MPa (including atmospheric pressure and water vapour pressure) at 10° C.), which will influence the effect of removal of hydroxyl radical scavengers, hence affecting utilization efficiency of ozone.
A good number of methods of improving the utilization efficiency of ozone in indirect reactions are available, such as catalysis of base, catalysis of metal catalyst, catalysis of hydrogen peroxide, catalysis of activated carbon, UV radiation, ultrasound, microwave, etc. These methods all focus on how to decompose ozone into hydroxyl radicals quickly and efficiently, but not to enhance the utilization efficiency of ozone from the perspective of the product from ozonation of the organic compounds in the wastewater or the removal of hydroxyl radical scavengers. In U.S. Pat. No. 6,913,698B2 “Method for reducing COD (chemical oxygen demand) in waste water by using O3 with valent ion chelation”, organic acid intermediates formed from the oxidization of organics with ozone will chelate with valent ions dissolved in water and precipitate from the waste water, so as to reduce the amount of ozone consumed and save cost. Appropriate valent ions in the patent encompass sodium ion, potassium ion, calcium ion, magnesium ion, aluminum ion, iron ion, chromium ion, titanium ion and niobium ion. Nevertheless, salts generated by lots of acids afforded by oxidization of organic compounds, especially the low-molecular-weight organic acids and these valent ions have relatively high solubility. For example, various salts of formic acid and acetic acid are substantially water-soluble, and they are difficult to be separated from water. Only calcium oxalate and ferrous oxalate among oxalates are of lower solubility. Calcium oxalate can be separated from water, and under a basic condition, ferrous ion is precipitated in a form of hydroxide which might be precipitated in a form of ferrous oxalate. The solubility of magnesium oxalate is 0.104 g/100 g water (20° C.). Only if the solubility product of oxalate ion and magnesium ion in water is greater than 8.57×10−5, can magnesium oxalate precipitate be yielded, to separate the oxalate ion from wasterwater.
In Chinese patents CN1699212A “Application of brucite in ozonization treatment of organic wastewater” and CN101157494A “Application of half-burning brucite in ozonization water purification and method thereof,” the pH of the system is balanced by adding brucite and other alkaline minerals to wastewater, thereby improving the oxidization efficiency of ozone. Here, brucite functions as a base, which is actually a base catalyst in the base-catalyzed treatment of wastewater by ozone. In Chinese patent CN103771650A “Method for treating coal gasification wastewater,” coal gasification wastewater is subjected to softening pretreatment by using lime, to conduct chemical precipitation on calcium salts and magnesium salts with lime milk in water to reduce the water hardness. After adjusting the pH value of the wastewater to 8.5˜10.0 by lime softening, the ozone oxidization treatment is carried out. In this patent, lime serves to remove calcium and magnesium ions and reduce water hardness as well as to adjust the pH of wastewater.
In Chinese patent CN102464422B “Method for pretreating industrial wastewater,” wastewater produced by a pharmaceutical industry is disposed of. After the treatment by iron-carbon micro-electrolysis and ozone oxidization process, a mixture of calcium oxide and calcium chloride is added for the aim of removing the sulfate ion in water and prevent the influence on the biochemical treatment of wastewater. In Chinese patent CN101164919A “Deep treatment method for garbage percolate,” after garbage percolate is oxidized by hydrogen peroxide or ozone, complexing agents containing calcium or barium are added to the oxidized solution, to make the organic intermediate products containing carboxyl group, carbonyl group or hydroxyl group generated by the oxidation precipitate under a basic condition and separated from water, hence reducing COD of wastewater. In this patent, the complexing agents containing calcium or barium serve to precipitate the organic intermediates after the oxidation.